Many years of research by the Bureau of Mines and others have shown that the mixing of a sufficient quantity of inert dust with coal dust will prevent coal dust explosions. Rock dust or limestone dust is the most common inert dust used. This inerting technique, referred to as rock dusting, is required by law in U.S. underground coal mines. At the present time, the coal dust-rock dust mixture is required by law to be at least 65% in all working areas (except the first 40 ft. from the face) and 80% in returns. In the presence of methane, the incombustible content of the dust must be increased by 0.4 and 1 pct., respectively, for each 0.1 pct. methane in the ventilating air.
In assuring compliance with the law, inspectors currently collect samples of deposited dust, in any given mine, every 200 ft. of entry once every 2 months. The conventional sample is a 6 inch wide band across the floor, ribs and roof to a depth of 2 inches, where possible. If the floor is well rock dusted but the roof and ribs are determined visually to be deficient in rock dust content, then it is recommended that the combined rib and roof portion of the band sample be kept separate from the floor portion so that a separate analysis can be run on each. The inspector screens the sample through a sieve and sends about 200 g of the sieved samples to Mt. Hope, WV for chemical analysis. The inspector also screens the sample through a No. 10 sieve, if possible, and also sends about 200 g of this sieved sample to Mt. Hope WV for analysis. The concentration of rock dust in the sample is obtained with volumetric methods by measuring the inverse in the volume of alcohol used in a volumeter. From this rock dust measurement, the incombustible content is computed. All borderline samples then undergo a more accurate low temperature ashing procedure.
Typically, the results of the analysis are received about 2 weeks after the sample is taken. In the meantime, the mine operators must rely on visual inspection (dark or light) of rock dusted areas to estimate the quality of the rock dusting practice on a day-to-day basis.
In the prior art, there have been various devices disclosed for detecting particle concentrations. For example, in U.S. Pat. No. 3,610,205 (Rarey), an apparatus for measuring and controlling the mixture content in an electrostatic printer is disclosed. This apparatus includes a window adjacent the path of movement of the mix of component particles which contacts the particles in transent therepast. The degree of reflected light depends upon the ratio of the component particles which differ in optical qualities.
In U.S. Pat. No. 3,872,825 (Davidson), an apparatus for detecting particle concentration in a mix of carrier granules and toner particles in a copying machine is disclosed. This apparatus includes a reflecting means which attracts toner particles thereto electrostatically. A beam of light rays is directed to the reflecting means and the concentration of the reflected beam indicates the concentration of toner particles 0 on the reflector surface which is a function of the toner particles in the mixture.
In U.S. Pat. Nos. 3,564,263 (Shaw) and 3,810,617 (Steinberg), apparatuses for determining the concentration of particles in a fluid stream are disclosed. The apparatuses use light rays which are directed through the stream.